The present invention relates to matched filters for correlation and convolution of information-carrying signals and is more particularly directed to optical matched filters.
Matched filters are used to extract useful information from electronic or optical signals. They find particular use in applications calling for high-speed signal processing where large amounts of analog or digital data need to be analyzed and processed in real time. Such applications include advanced radars, large sonar systems, communication networks, and real-time pattern recognition, to mention just a few. In a radar system, for example, a transmitter sends a signal that has been coded in a characteristic way in the direction of a target, and a receiver looks for a return signal reflected by the target. The return signal is often difficult to detect because it may be extremely weak, or buried in noise, or masked by various forms of interference, including intentional jamming. The matched filter operates by comparing the received signal with the transmitted signal and looking for the transmitted signal's code in the received signal. When a match is found, the delay between the transmitted and return signals may be determined and the range of the target may be calculated.
As another example, in certain applications it is necessary to search a database looking for a best match between a given signal pattern and the contents of the database. This need arises for example in searching a database of text looking for a particular alphanumeric string, or in searching a database of characteristic sonar echo waveforms representing the characteristic ways in which particular types of objects reflect a sonar signal.
In high-speed or high-volume signal processing applications, the filter must balance such factors as the signal processing speed, the quantity of data that may be manipulated at a time, the shear size of the database, the bandwidth needed to process signals, or even the power levels involved. In applications involving coded signals or large databases, changes in the code or in the database also require a corresponding update to the filter so that the ease with which the filter may be "reprogrammed" with the new codes or reference data also becomes a factor. In attempts to balance these factors, matched filters have been fashioned in the past based on a variety of technologies such as tapped delay lines, acousto-electric correlators, or acousto-photorefractive effects. These techniques, however, generally suffer from limited capacity for storing reference signals or generally lack rapid reprogramming capability for changing reference signals, which are vital in high-speed processing. An extensive discussion of the prior art as well as an example of an electronic matched filter are given in U.S. Pat. No. 4,633,426 of Venier issued Dec. 30, 1986.
In another line of development optical signal processing techniques have been employed for processing information-bearing optical signals as well as non-optical signals. To process non-optical signals, an optical beam, usually a laser beam, is first modulated with the information-bearing non-optical signal. The processing then proceeds by manipulating the modulated optical beam, and the resultant processed optical signal may then be detected and demodulated. An example of such a system is disclosed in U.S. Pat. No. 5,121,248 of Mohon et al. issued Jun. 9, 1992, which also provides an extensive discussion of prior art acousto-optic filters.
In parallel with the above has been the development of optical memory systems. Optical memory refers generally to a data storage system that utilizes the properties of a light beam to store data in a storage medium and to retrieve the data from the medium. Various types of optical memory systems have been devised utilizing various properties of a light beam. Conventional two-dimensional optical memories store data at geographically defined spatial addresses on the storage medium taking advantage of the ability of a light beam to be focused to a very small and precise spatial extent. Frequency-domain optical memories and coherent time-domain optical memories are examples of other types of optical memories that exploit, in addition, the frequency characteristics, or more generally the spectral characteristics, of the light beam to store and retrieve data at increased storage capacities. Coherent time-domain optical memories utilize the phenomenon of stimulated photon echo pulses and have been explored in the publications of T. M. Mossberg, Optics Letters, Vol. 77, p. 77 (1982); Y. S. Bai, W. R. Babbit and T. W. Mossberg, Optics Letters, Vol. 11, p. 724 (1986); J. M. Zhang, D. J. Gauthier, J. Huang and T. W. Mossberg, Optics Letters, Vol. 16, p. 103 (1991). In an article by Y. S. Bai, W. R. Babbitt, N. W. Carlson and T. W. Mossberg entitled "Real-time optical waveform convolver/cross correlator," Applied Physics Letters, vol. 45 (Oct. 1, 1984), pp. 714-716, the authors demonstrated the possibility of using the stimulated photon echo phenomenon to derive the cross-correlation and/or convolution of two optical signals. See also U.S. Pat. No. 4,459,682 of Mossberg et al. pertaining to that technique.